Time modulated stochastic screening

Abstract

A method for generating a screened reproduction of a multiple tone image comprising the steps of frequency modulation screening the multiple tone image to obtain screened data representing tones of the multiple tone image in terms of halftone dots, reproducing the halftone dots on an imaging element by means of a scanwise exposure, characterized in that the scanwise exposure for rendering a halftone dot is time modulated.

We claim: 
     
       1. A method for generating a screened reproduction of a multiple tone image comprising the steps of: frequency modulation screening said multiple tone image to obtain screened data representing tones of said multiple tone image in terms of halftones dots;   reproducing said halftone dots on an imaging element by means of a scanwise exposure;   wherein said scanwise exposure for rendering a halftone is time modulated by altering an exposure time for rendering halftone dots of low tones in one sense relative to halftone dots of mid tones, and altering an exposure time for rendering halftone dots of high tones in an opposite sense.   
     
     
       2. A method according to claim 1, wherein an exposure time for rendering halftone dots of low tones is lengthened relative to halftone dots of mid tones, whereas an exposure time for rendering halftone dots of high tones is shortened. 
     
     
       3. A method for generating a screened reproduction of a multiple tone image comprising the steps of: frequency modulation screening said multiple tone image to obtain screened data representing tones of said multiple tone image in terms of halftones dots;   reproducing said halftone dots on an imaging element by means of a scanwise exposure;   wherein said scanwise exposure for rendering a halftone is time modulated by varying an exposure time for rendering halftone dots in accordance with the number of neighbouring halftone dots.   
     
     
       4. A method according to claim 1, wherein said scanwise exposure comprises a single pulse exposure, the pulse-width thereof being modulated. 
     
     
       5. A method according to claim 1, wherein said scanwise exposure comprises a multiple pulse exposure, said time modulation being carried out by varying a number of pulses. 
     
     
       6. A method according to claim 1, wherein said scanwise exposure is carried out by an exposure unit moving across the imaging element during said scanwise exposure. 
     
     
       7. A method according to claim 1 wherein said imaging element is a lithographic printing plate precursor having a surface capable of being differentiated in ink accepting and ink repellant areas upon scanwise exposure and an optional development step. 
     
     
       8. A method according to claim 7 wherein said lithographic printing plate precursor contains a silver halide emulsion layer and an image receiving layer containing physical development nuclei and wherein subsequent to said scanwise exposure said lithographic printing plate is developed using an alkaline processing liquid in the presence of developing agent(s) and silver halide solvent(s).